The invention relates to an apparatus and a method for generating a reference frequency.
In a satellite TDMA network the various user stations need a common time and frequency reference for deriving their first transmission instance and their carrier frequency. Therefore, time and frequency synchronization of the user stations is an important feature which must respect severe constraints in order to lead to an efficient system with minimum interference between users and maximum throughput.
From WO 98/13969 time stamping clocks are synchronized in an ATM-network for compensating or measuring delays in the network. In an ATM-network the time value is sent in a field designated TSTP. The TSTP field consists in 32 bits and the time value is stored as the 32 least significant bits of the number of micro seconds that have passed since Jan. 1, 1972. By simply calculating the time difference of these values time stamping clocks can be run in each node which are phase-locked to the network synchronization clock. However, timing of the time stamping clocks can vary due to delays within the ATM-network. Delays in packet switched networks occur e.g. when switching a packet from one node in the network to another node. These delays can vary significantly and are due to the degree of utilization in the network and in individual nodes. Therefore, it is both necessary to measure and compensate the delays so as to enable a network to be trimmed or to ascertain which parts of the network are subjected to greater or smaller delays.
Firstly, in order to synchronize the time stamping clocks in different nodes these clocks are phase-locked to network-synchronization clocks and additionally, an absolute time is obtained from a GPS receiver. The GPS receiver supplies a TOD (Time Of Day) and PPS (Pulse Per Second) to a synchronization function. The synchronization function uses TOD to provide an absolute time for the time stamping clock in a node. There is thus obtained a synchronized absolute time between the time stamping clocks in the different nodes with a resolution of one second. The PPS pulse is used to obtain a degree of accuracy of one micro second between the clocks. Because the time stamping clocks are phase-locked to the network synchronization clocks and not to the GPS, the time stamping clocks will continue to have a high degree of accuracy even if the GPS equipment should malfunction or if the signal from the GPS satellite should be disturbed.
Secondly, delays are measured by time stamping a packet, i.e. by storing in the packet a value which represents the time at which the packet leaves the node. The packet is then sent to another node in the network and this node reads the stored value and compares the set value with the value on the time stamping clock in the own node. This provides a value of the delay.
From EP 0 564 220 A2 a clock synchronization system for synchronizing the performance of a number of clocks with a reference clock is disclosed. Each clock of this synchronization system includes a counter that indicates the current time and that is sequentially incremented by clock signal. A time, counter controller both initializes the counter and generates the clocking signal that controls the advancement of the counter. The time counter controller further monitors the time indicated by the counter and compares it to a reference-time signal received from a reference clock. Based on the comparison, the time counter controller selectively reinitializes the counter and adjusts a rate at which a clocking signal supplied to the counter so as to ensure that the counter advances at a rate equal to the rate at which the reference clock advances. Preferably, a reference-time signal is received from a global positioning system (GPS).
From EP 0 671 828 A1 a clock circuit is known to recover timing and transmitted information signals in a data receiver. Thereby, the accuracy of a low cost local master clock can be increased. The timing relationship can be derived from horizontal synch or colour burst pulses provided in a broadcast television signal. Another possibility of receiving a timing relationship is using a data packet communication scheme as digital HDTV data packet format or MPEG-2 format.
From EP 0 836 282 A1 an adapted phase-lock loop circuit is known for extracting time stamps from a MPEG-2 transport stream for obtaining a synchronized reference time. In a MPEG-2 transport stream a PCR (Program Clock Reference) time stamp is periodically transmitted. The PCR is detected from the transport stream by a PCR extracting circuit. When the time reference value is detected a counter counts a clock oscillated by a VCO (Voltage Controlled Oscillator) and a comparator compares the value of the counter and the value of the PCR. The phase difference between both values is fed back to the VCO through a digital filter. In the control start stage, the gain of the digital filter is designated to a large value. Thus, the phase difference is quickly converged to the allowable difference range. In the lock stage, the gain is designated to a small value. Thus, the control operation is stably performed.
For generating a time reference in a user station of a satellite TDMA network it has to be observed that the user station generally consists of an indoor unit (IDU) and an outdoor unit (ODU). The outdoor unit comprises a satellite antenna with an upconverter and/or a downconverter wherein the indoor unit comprises an encoder/decoder and a QPSK modulator. Having this separation-between an indoor unit and an outdoor unit in the past different mechanisms have been employed to achieve synchronization especially for satellite systems:
The IDU uses an xe2x80x9cexpensivexe2x80x9d Oven Controlled Oscillator (OCXO) as an extremely stable reference frequency. The IDU sends the signal at a fixed IF carrier frequency to the Outdoor Unit (ODU). The IDU also sends a reference frequency to the ODU, which is used by an xe2x80x9cexpensivexe2x80x9d microwave oscillator in the ODU in order to generate the upconverter frequency. The Indoor Unit (IDU) exchanges messages with the Hub in order to correct its IF frequency.
Same as in above, but the Outdoor Unit (ODU) uses another independent xe2x80x9cexpensivexe2x80x9d OCXO to have an extremely stable upconverter frequency. This avoids the use of a reference frequency sent from the IDU to the ODU.
The IDU locks its local oscillator to the down link signal, and exchanges messages with the Hub in order to generate the correct IF frequency. The IDU sends a reference frequency to the ODU, which is used by an xe2x80x9cexpensivexe2x80x9d, microwave PLL in the ODU in order to generate the upconverter frequency. This avoids the use of an expensive OCXO in the IDU.
In frequency agile systems, the IDU may also generate the IF signal directly in L-Band (959-2150 MHz), thus avoiding ODU upconverter frequency agility. The ODU upconverts the signal by a fixed oscillator frequency. This provides frequency agility but still requires an xe2x80x9cexpensivexe2x80x9d microwave PLL in the ODU in order to control the upconverter frequency.
From EP-A-0836282 an adapted phase lock loop circuit is known for extracting timestamps from a MPEG-2 transport stream for obtaining a synchronised reference time. The adapted phase lock loop comprises a voltage controlled oscillator (VCO) for generating an internal reference frequency.
From U.S. Pat. No. 5,841,987 a system for generating a signal for coupling digital audio, video and data signals in compressed form via a bus is known. A processing means formats the digital audio, video and data signals into super packets for transmission via the bus. Each super packet comprises a timestamp, wherein receiving devices utilise the timestamp for clock synchronisation. The realisation of the clock synchronisation is not explained in more detail.
From P. A. SARGINSON: xe2x80x9cMPEG-2: A Tutorial Introduction to the Systems Layerxe2x80x9d IEE COLLEQUIUM ON MPEG-2, 1995, pages 4/1-4/13 and 500 IN LEE ET AL.: xe2x80x9cImplementation of MPEG-2 TS Remultiplexer and Data Transport Unit for HDTV Satellite Broadcastingxe2x80x9d IEEE TRANSACTIONS ON CONSUMER ELECTRONICS, vol. 43, no. 3, Jun. 11-13, 1997, pages 324-329, a general description of the fundamental principles of the MPEG-2 systems layer is known, wherein some examples for conventional TV and HDTV are explained.
However, all these solutions involve high costs. Therefore, it is an object of the invention to generate a carrier frequency in a user station of a satellite TDMA network at low costs which is accurate and stable and which meets stringent phase noise requirements.
According to a first aspect of the invention, apparatus is provided for generating a reference frequency depending on a timing information extracted from a data stream. The apparatus comprises at least one free-running oscillation means, wherein at least two oscillation signals are derived from the at least one oscillation means, estimation means for estimating for each of the at least one free-running oscillation means the frequency deviation on the basis of the timing information, frequency modifying means for modifying the frequency of at least one of the at least two oscillation signals by a frequency correction value, linking means for linking the at least two oscillation signals to a signal yielding said reference frequency wherein the reference frequency is connected with the frequencies of the at least two oscillation signals by an unambiguous functional relation, and controlling means for determining the respective frequency correction value on the basis of the at least one estimated frequency deviation such that the timing information given by the reference frequency corresponds to the timing information extracted from said data stream.
According to a second aspect of the invention, apparatus is provided for generating a reference time depending on a timing information extracted from a data stream. The apparatus comprises a counter driven by a free-running oscillator and adjustable by counter step adjustment, wherein the counter step adjustment is controlled such that the content of the counter tracks the timing information within the data stream and yields the reference time.
According to a third aspect of the invention, a telecommunication network is provided. The telecommunication network comprises a master station and several base stations, wherein the master station transmits a time information in a data stream to each of the base stations and each of the base stations comprises an apparatus for generating a reference frequency depending on a timing information extracted from a data stream, further comprising: at least one free-running oscillation means, wherein at least two oscillation signals are derived from the at least one oscillation means, estimation means for estimating for each of the at least one free-running oscillation means the frequency deviation on the basis of the timing information, frequency modifying means for modifying the frequency of at least one of the at least two oscillation signals by a frequency correction value, linking means for linking the at least two oscillation signals to a signal yielding said reference frequency wherein the reference frequency is connected with the frequencies of the at least two oscillation signals by an unambiguous functional relation, and controlling means for determining the respective frequency correction value on the basis of the at least one estimated frequency deviation such that the timing information given by the reference frequency corresponds to the timing information extracted from said data stream.
According to a fourth aspect of the invention, a method is provided for generating a reference frequency depending on a timing information extracted from a data stream. The method comprises the steps of: providing at least two oscillation signals derived from at least one free-running oscillation means, estimating for each of the at least one free-running oscillation means the frequency deviation on the basis of the timing information, modifying the frequency of at least one of the at least two oscillation signals by a frequency correction value, linking the at least two oscillation signals to a signal yielding said reference frequency wherein the reference frequency is connected with the frequencies of the at least two oscillation signals by an unambiguous functional relation, and determining the respective frequency correction value on the basis of the at least one estimated frequency deviation such that the timing information given by the reference frequency corresponds to the timing information extracted from said data stream.
According to a fifth aspect of the invention, a method is provided for generating a reference time on the basis of timing information extracted from a data stream. The method comprises the steps of: driving a counter having an adjustable counter step adjustment by a free-running oscillation means; controlling the counter step adjustment such that the content of the counter tracks the timing information within the data stream; and using the content of the counter as said reference time.
According to a sixth aspect of the invention, a method is provided for estimating the frequency deviation of a free-running oscillation means on the basis of timing information extracted from a data stream. The method comprises the steps of: driving a counter having an adjustable counter step adjustment by the free-running oscillation means; controlling the counter step adjustment such that the content of the counter tracks the timing information within the data stream; and using the counter step adjustment as an estimation of said frequency deviation.
The inventive solution is based on the cognition that within the user stations at least one free-running oscillator is provided from which at least a local oscillator (LO) frequency and an intermediate frequency (IF) are derived. It is possible to derive both the LO-frequency and the IF-frequency from one single free-running oscillator or to derive the LO-frequency from a first free-running oscillator and the IF-frequency from a second free-running oscillator.
Any deviation of the actual frequency of the free-running oscillator from its nominal value causes a deviation of the carrier frequency. Therefore, the idea of the invention is to estimate the frequency deviation of the free-running oscillator by comparison with a timing information extracted from a data stream and to modify the IF-frequency by a frequency correction value. The IF-frequency and the LO-frequency are linked by a linking means to a radio frequency (RF) wherein a controlling means determines the respective correction value on the basis of the estimated frequency deviation such that the timing information given by the reference frequency corresponds to the timing information extracted from said data stream.
Obtaining the RF-frequency as a reference frequency a reference time can be generated by a counter which is driven by the RF-frequency wherein the content of the counter yields the reference time.
Another possibility for generating a reference time depending on a timing information extracted from a data stream is to track the timing information of the data stream by a counter which is driven by an oscillator within a closed loop circuit wherein the content of the counter yields the reference time.
The invention can be applied to a telecommunication network comprising a master station and several base stations wherein the master station transmits a time information in a data stream to each of the base stations and wherein each of the base stations comprises an apparatus for generating a reference frequency according to the invention.
An apparatus and a method formed in accordance with the invention have a number of advantages. Hence, according to the invention the need for an expensive ODU oven controlled oscillator can be avoided. Furthermore, the synchronization of the carrier frequency is not required by the master station. Eventually, frequency and timing information to all base stations or user stations can be delivered on the basis of an MPEG accompliant mechanism avoiding the need for additional time synchronization mechanisms such as start of frame transmission or other specific mechanisms which are not defined by MPEG. Furthermore, if several indoor units are connected to one outdoor unit cost savings can be achieved by placing one single oscillator in the outdoor unit.
In accordance with one aspect of the present invention the timing information consists of time stamps transmitted within the data stream. Preferably the time stamps consist of a program clock reference (PCR) of a MPEG-2 transport stream based on a 27 MHz clock.
In accordance with another aspect of the present invention the linking means consists of at least one mixer whose output signal has a frequency which is a linear combination of the frequencies of the respective input signals. Usually, the mixer is combined with a filter such that the output signal of said mixer amounts to the sum of the frequencies of the input signals. However, it is also possible to design the filter in combination with the mixer such that the frequency of the output signal of said mixer amounts to the difference of the frequencies of the input signals.
In accordance with a further aspect of the present invention the estimation means estimates the frequency deviation of the respective oscillation means according to the formula       frequency    ⁢          xe2x80x83        ⁢    deviation    =                                          t            inf                    ⁡                      (            data            )                          -                              t            inf                    ⁡                      (            osc            )                                                t          inf                ⁡                  (          data          )                      ·          frequency      ⁡              (        osc        )            
with tinf(data): timing information of data stream
tinf(osc): timing information of oscillation means
frequency(osc): frequency of oscillation means
Advantageously, the respective timing information is referred to the period after applying the last correction by the frequency modifying means, i.e. any time measurement for obtaining the timing information starts after the last correction by the frequency modifying means.
In accordance with another aspect of the present invention the estimation consists of a counter whose content tracks the timing information of the data stream within a closed loop circuit wherein the adjustment step for the counter serves as an estimation of the respective frequency deviation. Advantageously, the content of the counter can also be used as a reference time signal.
In accordance with a further aspect of the present invention the estimation comprises a phase-locked loop generating a PLL-frequency on the basis of the timing information wherein the frequency deviation is obtained by comparing the PLL-frequency with the frequency of the respective oscillation means.
In accordance with another aspect of the present invention the frequency modifying means consists of a frequency scaling means modifying the frequency of the respective oscillation signal by a scaling factor.
In accordance with a further aspect of the present invention the controlling means consists of a micro processor. For obtaining the frequency of the at least one oscillation means an AD-converter is provided. The AD-converter simply can consist of a comparator transferring the sinusoidal signal of the oscillation means in a square-wave signal which can easily be processed by the micro processor. If necessary, a frequency divider can be additionally provided in order to transfer the frequency of the at least one oscillation means to a lower frequency being more suitable with regard to the processing speed of the micro processor. To determine the deviation between the program clock reference of the MPEG-2 transport stream and the at least one oscillation means the transmitted PCR time stamp is periodically processed by the micro processor. There should be a constant ratio between the frequency of the at least one oscillation means and the program clock reference. If not, the micro processor has to determine the respective frequency correction value based on the difference between the internal frequency of the at least one oscillation means and the PCR value. Hence, also the estimation means can completely be implemented by software in the micro processor yielding a very simple and adaptable apparatus.
In accordance with another aspect of the present invention the at least one oscillation means consists of a first free-running oscillator. This implementation allows a most inexpensive realization of the invention. Furthermore, it has to be noted that in case of several indoor units are connected to one outdoor unit cost savings can be achieved in placing the single oscillator within the outdoor unit. Preferably, the frequency of the first free-running oscillator lies below the IF-frequency such that the IF-frequency is yielded by increasing the frequency of a first oscillation signal derived from the first free-running oscillator. Additionally, a second oscillation signal is derived from the first free-running oscillator which is fed to a frequency multiplier in order to provide an LO-frequency. The actual RF-frequency can be derived by mixing the IF-frequency with the LO-frequency. Due to the correction within the controlling means the frequency deviations of the first oscillation signal and the second oscillation signal derived from the first free-running oscillator are compensated such that the RF-frequency is a reference frequency with regard to the time reference received by the PCR values.
In accordance with another aspect of the present invention in addition to the first free-running oscillator a second free-running oscillator is provided yielding a third oscillation signal. The third oscillation signal is fed to the frequency modifying means yielding the IF-carrier signal wherein the second oscillation signal is still fed to the estimation means. Hence, the third oscillation signal is modified by the frequency modifying means such that the frequency deviations of the first free-running oscillator and the second free-running oscillator are compensated.
In order to transmit a payload signal or a user signal the IF-carrier signal is fed together with the user signal to a mixer yielding an intermediate transmission signal which is fed together with said second oscillation signal to a mixer yielding a RP-signal with a carrier frequency corresponding to said reference frequency.